1. Field of the Invention
The present invention relates to a process chamber for semiconductor wafers, and, more particularly, to a method and apparatus for processing semiconductor wafers wherein gases can be supplied to both sides of a susceptor which supports the semiconductor wafer during processing and to different areas of the surface of the semiconductor wafer.
2. Background of the Invention
One type of processing apparatus for semiconductor wafers is a single wafer processor in which one wafer at a time is supported on a susceptor in a processing chamber. The susceptor divides the chamber into one portion which is below the susceptor (the lower portion), and a second portion which is above the susceptor (the upper portion). The susceptor is generally mounted on a shaft which rotates the susceptor about its center to achieve a more uniform processing of the wafer. A flow of a processing gas, such as a deposition gas, is provided in the upper portion of the chamber and across the surface of the wafer. The chamber generally has a gas inlet port at one side thereof, and a gas outlet port at an opposite side to achieve a flow of the processing gas across the wafer. The susceptor is heated in order to heat the wafer to a desired processing temperature. One method used to heat the susceptor is by the use of lamps provided around the chamber and directing their light into the chamber and onto the susceptor. In order to control the temperature to which the wafer is being heated, the temperature of the susceptor is constantly measured. This is often achieved by means of an infrared temperature sensor which detects the infra-red radiation emitted from the heated susceptor.
One problem with this type of processing apparatus is that some of the processing gas, which is often a gas or mixture of gases for depositing a layer of a material on the surface of the wafer, tends to flow around the edge of the susceptor and deposits a layer of the material on the back surface of the susceptor. Since the deposited material is generally different from the material of the susceptor, the deposited layer has an emissivity which is different from that of the emissivity of the susceptor. Thus, once the layer of the material is deposited on the back surface of the susceptor, the infrared temperature sensor detects a change caused by the change in the emissivity of the surface from which the infra-red radiation is emitted. This change indicates a change in temperature of the susceptor which actually does not exist.
One technique which has been used to prevent the problem of deposits on the back surface of the susceptor is to provide a flow of an inert gas, such as hydrogen, into The lower portion of the chamber at a pressure slightly greater than that of the deposition gas in the upper portion of the chamber. One apparatus for achieving this is described in the application for U.S. Patent of Roger Anderson et al., Ser. No. 08/099,977, filed Jul. 30, 1993, entitled "Gas Inlets For Wafer Processing Chamber". Since the inert gas in the lower portion of the chamber is at a higher pressure, it will flow around the edge of the susceptor from the lower portion of the chamber into the upper portion of the chamber. This flow of the inert gas prevents the flow of the deposition gas into the lower portion of the chamber. Although this is very satisfactory for preventing deposition of a layer from the deposition gas on the back surface of the susceptor, it does have a disadvantage. The flow of the inert gas into the upper portion of the chamber at the edge of the susceptor dilutes the deposition gases at the edge of the susceptor. This, in turn, causes a non-uniformity of the composition of the deposition gas so as to adversely affect both the thickness and resistivity uniformity of the layer being deposited.
Another technique which has been developed to limit the flow of inert gas around the edge of the susceptor from the lower portion of the chamber to the upper portion of the chamber is to overlap the edge of the susceptor with a preheat ring which surrounds the susceptor. This forms a labyrinth passage which greatly reduces the flow of gases around the edge of the susceptor. Such an apparatus is described in the application for U.S. Patent of Israel Beinglass, Ser. No. 08/090,591, filed Jul. 13, 1993, entitled "Improved Susceptor Design". Although this reduces the flow of gases around the edge of the susceptor, it has a disadvantage that the susceptor and preheat ring must be aligned very accurately so that the gap therebetween is small enough to prevent flow of the processing gas therethrough, but large enough so that the susceptor does not scrape or rub on the preheat ring. If the susceptor scrapes or rubs against the preheat ring, it would cause the formation of particles which can contaminate the layer being deposited, or cause mechanical damage to the susceptor. This spacing between the susceptor and preheat ting is difficult to maintain because of the mechanical tolerance stack-up of parts and because of other factors, such as thermal expansion or wear-in.
Therefore, it would be desirable to have a method and apparatus Which overcomes the problem of improper indication of temperature resulting from a coating applied to the back surface of the susceptor. Also, it would be desirable to have an apparatus which overcomes the problem of dilution of the deposition gases by the flow of inert gas around the susceptor form the lower portion of the chamber.